The present invention relates to a power supply apparatus; and more particularly, to a compact power supply apparatus in which a transformer and other components are small and sufficiently thin to be suitable for use as a power supply for a portable information machine, such as a notebook type word processor or a personal computer.
In a conventional multi-output power supply of the type used in a hand held compact information machine a switching regulator such as shown, for example, in Japanese utility model laid-open No. 64988/1992, is widely used. In this switching regulator, a direct current input is applied to a primary coil of a transformer and a switching element, which are connected in series. (The transformer has a multi-coil form and plural secondary coils at a secondary side.) The switching element switches "on" and "off", so that at the plural secondary coils of the transformer an alternating current having the same frequency as the frequency of the switching element can be obtained. The alternating current is then rectified and supplied to a load.
As a control of an output voltage in the above-stated conventional machine, it has been generally known that, the pulse width of the switching element at the primary side of the transformer is varied in response to increases and decreases of the main load voltage.
In hand held information machines, it is important for the machine to have a compact size and a thin power supply. However, one difficulty posed by such power supplies is the compact size of the transformer used in the power supply portion.
By using a transformer having no iron core, such as wire type transformer having a parallel arrangement of a primary coil and a secondary coil which are insulated from each other, contacted and wound spirally, or by using a thin transformer in which a thin iron core is added to the above-stated wire-type transformer arrangement, it is possible to provide an extremely thin transformer, with a thickness of about 2-5 mm.
However, since it is intended to obtain plural outputs from the above-stated thin transformer, it is necessary to have plural insulated secondary coils, which presents a problem for obtaining a thinner construction of the transformer.
An object of the present invention is to provide a compact, thin power supply apparatus without use of plural transformers, and without complicated circuit construction.
To attain the above-stated object, the power supply apparatus according to the present invention comprises a transformer having a primary coil and a secondary coil, at least one switching element connected in series to the primary coil of the transformer, a first rectifying and smoothing circuit connected to the secondary coil of the transformer, and a secondary rectifying and smoothing circuit connected to the secondary coil of the transformer.
A first output is obtained from the first smoothing circuit and a secondary output is obtained from a capacitor of the second smoothing circuit. The present invention permits the circuit to be operated at a 70% maximum duty rate of the switching element. As used herein, the term "duty" rate refers to that portion of a single cycle during which the switch is in an "on", or conducting state.
According to the present invention, a supply voltage is applied to the primary coil of the transformer, and one or more output voltages are output through the secondary coil of the transformer. The value of the one or more output voltages differs from that of the supply voltage.
The transformer is comprised of overlapping substantially planar coils which are wound spirally, with separate conductors, each conductor having substantially the same length. One or more coils are formed as the primary coil of the transformer, and one or more coils are formed as the secondary coil. In a preferred embodiment, at least one part of the plural coils constituting the transformer is covered by an iron core made from a ferromagnetic body.
At an "on" time of the switching element, a first output is obtained from a current flowing in the first rectifying and smoothing circuit, and at an "off" time of the switching element, a second output is obtained from a current flowing in the second rectifying and smoothing circuit.
The D.C. voltage input from the direct current power supply is applied to the primary coil of the transformer during the "on" state of the switching element. During this time, the current is supplied to the smoothing circuit via the first diode connected to the secondary coil of the transformer and the direct current electric power is supplied to the first load.
When the above-stated switching element is switched to the "off" state, the reset current which constitutes the excitation energy of the transformer is supplied to a closed circuit which includes a second diode and a capacitor, which are also connected to the secondary coil of the transformer. During this time, a direct current voltage is generated in response to the electrical charge flowing into the capacitor. This D.C. voltage is smoothed by the output voltage stabilizing circuit and D.C. electric power is provided to the second load.
By means of the above-stated circuit construction, at the reset time of the transformer, the voltage accumulated in the above-mentioned capacitor is applied to the secondary side of the transformer, thus inducing a voltage at the primary side, which may actually be greater than the input voltage.
In conventional forward type convertors, the reset time of the transformer requires at least 50% in each cycle. However, according to the present invention, the reset time of the transformer is shortened to less than 30%, and it is thus possible to increase the "on" cycle of the switching element to a more than 70% duty rate. As a result, the winding ratio between the primary side and the secondary side of the transformer can be made smaller than that of the conventional machine, and the transformer volume can be reduced. In addition, the copper loss in the coil itself be reduced.
According to the present invention, by utilizing the excitation energy accumulated in the transformer such as the wire-type transformer, in addition to the output voltage conventionally obtained from the secondary side of the transformer, the output voltage can also be obtained through a simple second circuit arrangement connected to the secondary side of the transformer.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.